1. Field of the Invention
The present invention relates to image forming technologies such as copiers, printers, and facsimiles that form color or monochrome images by using image forming methods such as electrophotography, electrostatic recording, ionography, and magnetic recording and more particularly to a method and device for estimating toner concentration from amounts of toner transferred onto a photosensitive medium and other data and an image forming apparatus equipped with such device.
2. Prior Art
Electrophotographic image forming apparatuses that develop and visualize electrostatic latent images formed on the photosensitive medium, employing a two-component developer (developing agent) consisting of carrier and toner, are known.
Since only the toner is consumed during the image forming process of the image forming apparatuses employing the two-component developer, the toner in the developer decreases as the image forming process is repeated and the toner concentration gradually decreases. For countermeasures, the image forming apparatuses employing the two-component developer are arranged to include a toner concentration detector and detect toner concentration at appropriate intervals in order to maintain the toner concentration in the developer within a predetermined range. Additional toner is soon supplied when it is detected that the toner concentration has decreased less than the predetermined range.
For methods by which the toner concentration detector may operate, the following two methods are known: projecting light onto the developer loaded in a developing unit and detecting the toner concentration, based on the intensity of reflected light (optical ATDC); and detecting the magnetic permeability of the developer loaded in the developing unit and detecting the toner concentration, based on the magnitude of the permeability (magnetic ATDC).
Because the toner concentration detector is needed per developing unit, as many toner concentration detectors as the number of developing units are required for image forming apparatuses with a plurality of developing units, especially, like color image forming apparatuses, and this is a factor in increasing the cost of manufacturing these apparatuses. To solve this problem, diverse methods have been proposed.
A fist method is to control toner supply in this way: forming toner patches for adjusting toner concentration on the photosensitive medium with the same color and at least two different developing bias voltages; detecting the toner image density of the formed toner patches by toner image density sensors; and, from the results of the detected toner image density of two toner patches, detecting that the toner image density falls more than a specified value (refer to Japanese Laid Open Patent Publication No. 162795/2002).
A second method is to control toner supply in this way: forming the toner patches for adjusting toner concentration with a starting developer having a known toner concentration; detecting the amount of toner transferred; calculating development efficiency (change in the amount of toner transferred as the developing bias changes); determining compensation data by comparing the calculated development efficiency with the standard development efficiency of the starting developer; compensating the calculated development efficiency with the compensation data; and estimating the toner concentration in the developer (refer to Japanese Laid Open Patent Publication No. 248750/1996).
A third method, which uses both optical and magnetic toner concentration detectors to detect toner concentration, is to control black toner supply in this way: equipping a cyan developing unit for color image forming with an optical toner concentration detector and a black developing unit with a magnetic toner concentration detector; and comparing the standard development efficiency of the detected cyan toner concentration and the standard development efficiency of the detected black toner concentration (refer to Japanese Laid Open Patent Publication No. 106168/1997).
The foregoing first method only determines whether toner concentration falls within its upper and lower limits and is effective for those developers that have a sufficiently broad control range (operating window) of toner concentration, but has a disadvantage that it cannot provide accurate control to keep the toner at a constant concentration for developers with a narrow control range of toner concentration.
To explain the foregoing second method, FIGS. 15 and 16 show relationships between developing bias voltages and detected amounts of toner transferred for different levels of toner concentration. FIG. 15 shows that the amount of toner transferred changes slightly as the developing bias voltage changes in the case of low toner concentration (line L), whereas the amount of toner transferred changes largely as the developing bias voltage changes in the case of high toner concentration (line H) FIG. 16 shows that the amount of toner transferred changes slightly as the developing bias voltage changes, whether the toner concentration is high or low.
Toner concentration can be estimated from the toner patches if a large change in the development efficiency (change in the amount of toner transferred as the developing bias changes) occurs in conjunction with change in the toner concentration, as shown in FIG. 15. But, the second method has a disadvantage that it is impossible to estimate toner concentration if a small or little change in the development efficiency (change in the amount of toner transferred as the developing bias changes) occurs in conjunction with change in the toner concentration, as shown in FIG. 16.
Furthermore, the foregoing third method uses two types of toner concentration detectors based on different methods of toner concentration detection and its disadvantage is that a plurality of detectors with different properties are required.